CBM Potential of the Drumheller Coal Zone in the Alix Area
(Township 39-40; Range 22-23W4)

This Alberta Geological Survey paper reports on the potential for coalbed-methane (CBM) production of the Drumheller Coal Zone in an area comprising four townships near Alix, Alberta (Figures 1 and 2). Click on a thumbnail picture to load a larger version.

Figure 1 Isopach of cumulative coal in the Lower Horseshoe Canyon formation (from McCabe et al., 1989) (File size: 109 KB)

Figure 2 Location map of the Alix area with location of wells used in study and lines of cross sections (File size: 76 KB)

The results may also assist in locating a pilot site for enhanced coalbed methane recovery by injecting carbon dioxide (CO₂). This area was selected for detailed analysis using criteria of maximum cumulative coal thickness, optimal depth to the coal zone (between 300 to 600 metres) and proximity to sources of CO₂, using data from McCabe et al. (1989).

Definition of the Drumheller Coal Zone evolved since the work of Dr. John Allan on the Drumheller coalfield in the 1920s and 1940s (Allan, 1922; Allan and Sanderson, 1945). More recent contributions by Alberta Geological Survey include a resource assessment report of this coal zone by McCabe et al. (1989) and a regional assessment of Alberta Plain's CBM by Beaton et al. (2002). The Drumheller Coal Zone can be subdivided into two distinct zones - the Upper and Lower Drumheller Coal zones.

The Drumheller Coal Zone defines the lower part of the upper Cretaceous Horseshoe Canyon Formation. Because the strata dip toward the mountains, the Drumheller Coal Zone is exposed along the eastern and northern edges of the Horseshoe Canyon Formation outcrop. The Horseshoe Canyon Formation is underlain by the Bearpaw Formation and overlain by the Battle Formation. The thin Whitemud Formation, which has been mapped in outcrop between the Battle and Horseshoe Canyon formations, could not be recognized in the subsurface and is considered part of the Horseshoe Canyon Formation in this discussion. The Battle Formation was encountered only in wells in the western part of the study area (the formation surfaces in the eastern part of the study area) and is about seven metres thick.

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Bearpaw Formation

The Bearpaw Formation consists largely of dark and brownish-grey marine, partly silty shales, but fine- to medium-grained sandstones are also present. A complete core of the Bearpaw Formation is available from the nearby Alberta Research Council Castor well at 13-34-37-13W4 (classified as non-system core by the ERCB Core Research Centre; see also Given and Wall, 1971). In this well, the formation consists of an alternating series of sands and shales with a thickness of 143 metres. There are two sandstone and three shale units present; one of these shale units is probably the shale unit we used as a marker in the present study (Figure 3). Another complete core through the Bearpaw Formation has been collected from the C.P.O.G. Strathmore 7-12-25-25W4 well near Calgary (Hamblin, 1998; Hamblin, in press).

On Battle River and Paintearth Creek (east of the study area), the Bearpaw Formation is about 119 metres thick and the same members as in the nearby Castor well (13-34-37-13W4, described by Given and Wall, 1971) can be recognized. The sections of the Bearpaw Formation, exposed along the Red Deer River east of Drumheller near Dorothy, comprise the upper 70 metres of the formation and consist of shale, the Dorothy Bentonite and the Dorothy Sandstone (Given and Wall, 1971).

Figure 3. Structural map of the base of a prominent shale unit in the Bearpaw Formation (File size: 115 KB)

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Horseshoe Canyon Formation

The overall lithology consists of mudstone and siltstone with occasional sandstone bodies and coal. Other lithologies, such as bentonite and concretions, are thin, frequent, but laterally less extensive. In the Alix area, the structural map of the base of Horseshoe Canyon Formation has a general north-south trend and a westerly dip of 4.5 m/km (as shown by the structure of the base of a prominent shale unit in the Bearpaw Formation in Figure 3).

The Horseshoe Canyon-Battle Formation contact is abrupt on gamma-ray logs, suggesting a sudden change in sedimentation from a coarsening-upward succession in the Horseshoe Canyon Formation to a muddy sequence in the Battle Formation. The contact appears to be erosional.

The lower part of the formation contains thick coal seams and defines the Drumheller Coal Zone. For the Alix area, the base of the Horseshoe Canyon Formation is arbitrarily defined at the base of the uppermost shale horizon of the Bearpaw Formation, indicated by a particular gamma-ray and resistivity signature. The contact between the Bearpaw and Horseshoe Canyon formations is intertonguing and is related to a regression of the Bearpaw sea.

The Drumheller Marine Tongue of the Drumheller (Hamblin, in press) separates the upper from the lower Horseshoe Canyon Formation, but cannot be recognized in the subsurface. It might be equivalent to the top of the Drumheller Coal Zone.

In the western part of the study area, a few wells show the entire succession of the Horseshoe Canyon Formation, which is about 400 metres thick (13-05-40-23W4 has a thickness of 412 m; 13-29-39-23W4 has 394 m; and 08-18-39-23W4 has 398 m).

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Drumheller Coal Zone

The Drumheller Coal Zone comprises the thick coal seams of the Drumheller area at the base of the Horseshoe Canyon Formation. The top of the Drumheller Coal Zone is assumed to be above the highest coal seam of the zone, which is somewhat arbitrary. There might be some coal at a stratigraphic level equivalent to the Weaver Coal Zone in the Alix area; however, these correlations are still uncertain, and for now, these coals are included in the Drumheller Coal Zone inventory. The wells in the western part of the area, that contain a complete section of the Horseshoe Canyon Formation, show the Carbon/Thompson coal seams of the upper Horseshoe Canyon. Coal and shaly coal seams form up to four-metres thick seams with low density and gamma-ray values.

Figure 4 shows a typical geophysical log of the Drumheller Coal Zone of the Alix area. A prominent coarsening-upward succession could be correlated across the study area and the top of this unit has been named CU1. The base of this coarsening-upward unit is the E Marker of McCabe et al. (1989).

Figure 4. Typical geophysical logs for the Drumheller Coal Zone (File size: 202 KB)

Figure 5. J-J’ East-West cross-section through the Alix area. Line of cross-section is shown on Figure 2 (File size: 258 KB)

Figure 6. T-T’ North-South cross-section 1 through the Alix area. Line of cross-section is shown on Figure 2 (File size: 264 KB)

Figure 7. K-K’ North-South cross-section 2 through the Alix area. Line of cross-section is shown on Figure 2 (File size: 285 KB)

Figure 8. W-W’ North-South cross-section 3 through the Alix area. Line of cross-section is shown on Figure 2 (File size: 291 KB)

Four structural cross-sections through the Alix area are shown in Figures 5, 6, 7 and 8. They show the Bearpaw shale marker, the CU1 marker and details of the correlation of the coal seams and their frequent lack of continuity. The elevation of the CU1 marker (Figure 9) shows a very regular five m/km west-dipping surface.

The depth to the top of the coal zone increases from northeast to southwest from 160 to 360 metres (Figure 10).

Figure 9. Structural map of the top of a prominent coarsening-up unit (CU1) in the Drumheller Coal Zone (File size: 100 KB)

Figure 10. Depth to top of Drumheller Coal Zone (File size: 137 KB)

This surface is very irregular because of lack of continuity of individual coal seams, making this surface an arbitrary stratigraphic contact. Having no physical meaning, the only value of this surface is the approximate minimum depth to the coal targets. Total cumulative coal thickness ranges between s6 metres and 20 metres. The thickest area defines an angular belt in the southwest corner of the map. It covers 3/4 of the map in Township 39 along Ranges 22-23 and Township 40, Range 23 (Figure 11).

Figure 11. Drumheller Coal Zone cumulative coal (thickness in metres) (File size: 116 KB)

The north-south and northwest-southeast thickness trends in this map are mainly structural in origin and correspond with subsidence areas that favoured peat accumulation. The north-south subsidence directions were active during Drumheller deposition. Consequently, coal accumulation shows this preference. The number of coal seams ranges from 6 to 28. The maximum concentration of coal seams is in the western half of the map, mostly along Range 23, Townships 39-40 (Figure 12). The western part of the study area contains the best coal deposits. This is also indicated by the thickest coal seam map (Figure 13), whereby a coal seam includes both coal and shaly coal.

Figure 12. Number of coal seams in the Drumheller Coal Zone (File size: 138 KB)

Figure 13. Isopach of thickest coal seam of the Drumheller Coal Zone (File size: 128 KB)

The easiest correlatable coal seam of the study area is the thick coalbed above the CU1 coarsening-upward marker. This seam splits in two parts in the southern part of the area. The identification of this coal marker is based on thickness, stratigraphic position and location right above the thickest coarsening-upward sequence. Genetically, the marker coal belongs to the Lower Drumheller Coal Zone, but for coalbed methane assessment, this seam has been included within the Upper Drumheller Coal Zone because the CU1 marker divides the coal zone in an upper and lower part.

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Lower Drumheller Coal Zone

The Lower Drumheller Coal Zone ranges in thickness from 67 to 98 metres in the study area and consists of delta plain sediments with coarsening-upward sequences. Cumulative coal of the Lower Drumheller Coal Zone ranges from two to eight metres (Figure 14). This map shows two north-south-trending zones of thicker peat deposition, possibly indicating areas of larger subsidence. In the north part of the study area, the west-east direction represents an area of least coal accumulation. The zone consists of one to nine coal or shaly coal seams. The least thickness defines an isolated area within Township 40, Range 23, north central part.

Cumulative sand deposition during lower Drumheller time is shown in Figure 15, showing a similar north-south deposition direction as the peat deposition.

Figure 14. Lower Drumheller Coal Zone cumulative coal (thickness in metres) (File size: 141 KB)

Figure 15. Cumulative thickness of sandstone in the Lower Drumheller Coal Zone (File size: 120 KB)

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Upper Drumheller Coal Zone

The Upper Drumheller Coal Zone represents the stratigraphic interval between the top of the CU1 marker and the uppermost coal seam of the package. This zone represents sediments deposited on a flood plain between channels, as indicated by fining-upward sequences. The thickness of this zone ranges between 40 and 75 metres. Cumulative coal of the Upper Drumheller Coal Zone ranges from 3 to 15 metres (Figure 16) and shows both north-south and northwest-southeast-oriented thickness trends, possibly indicating preferred subsidence in the west, southeast and northeast. It appears that northwest-southeast-directed subsidence overprints a still active north-south-directed subsidence. The zone consists of 2 to 13 coal or shaly coal seams.

Cumulative sand deposition during upper Drumheller time is shown in Figure 17. This map shows two directions of distribution: north-south in the western part connected with a northwest-southeast direction in the central part.

Figure 16. Upper Drumheller Coal Zone cumulative coal (thickness in metres) (File size: 155 KB)

Figure 17. Cumulative thickness of sandstone in the Upper Drumheller Coal Zone (File size: 108 KB)

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Sedimentary Environments of the Drumheller Coal Zone

The sedimentological setting of the Drumheller Coal Zone is reviewed by McCabe (1989) and Hamblin (in press). In the study area and surrounding vicinity, lithological descriptions of core or cuttings are not available for the Drumheller Coal Zone. Consequently, the present facies interpretation is based on logs. The gamma ray and resistivity logs are sensitive indicators of sand/mud variations, and thus, ideally suited for the identification of fining- and coarsening-upward cycles.

In the study area, the depositional interpretation was made based on the identification of three main sedimentological elements: coarsening-upward, fining-upward and vertical aggradation with no specific upward trend.

The lower Horseshoe Canyon Formation (Drumheller Coal Zone) has gamma-ray signatures characteristic of a large inter-channel depositional area. Some differences are apparent between a lower and an upper part of the Drumheller Coal Zone. Both the lower and upper Drumheller sequences are cyclic. The top of a cycle is usually represented by coalbeds, which cap either the coarsening-upward sequences within the lower Drumheller Coal Zone or the fining-upward sequences within the upper Drumheller Coal Zone.

The depositional history shows two stages, which can be separated based on the level of depositional energy: The Lower Drumheller Coal Zone represents relatively high energy levels and the Upper Drumheller Coal Zone reflects lower energy levels.

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The Lower Drumheller Coal Zone (Upper Delta Plain Sequence)

The Lower Drumheller Coal Zone has frequent coarsening-upward sequences, interpreted as short channels and crevasse splays related to major channels, likely located outside the study area. The geophysical logs of the Lower Drumheller Coal Zone show interlaminated and interbedded siltstone and fine- to medium-grained sandstone in relatively thin (<5 m), aggradated successions, which are bounded above by coal or mud beds. The interbedded, heterolithic character of these facies associations is reflected by a 'spiky' character of gamma-ray curves through this succession. The 'spiky' character is interrupted by a 'blocky,' decreasing-upward gamma-ray trend, reflecting coarsening-upward grain size (Figure 4).

The base of coarsening-upward sequences shows a gradational increase of grain size. The top is sharply truncated by flat laminations of soil or coalbeds. Occasionally, medium-grained sandstone occurs within sharp-based, sheet-like beds, several meters thick, which are interpreted as minor channels. Coarsening-upward successions are interpreted as the progradation of minor channel systems into brackish and/or freshwater swamps.

These log signatures correspond to upper delta plain cycles with rhythmic gradation to peat accumulations at the top of the cycles. Part of the delta in the Alix area may also be tidally dominated, as documented by Rahmani (1988) in the Drumheller area. The upper delta plain sequence encompasses the uppermost part of the latest local Bearpaw marine transgression (uppermost Bearpaw marine shale), up to the last strong sediment supply pulse (unit CU1), including the overlying main coal marker. The sequence marks the transition from the marine (Bearpaw) to the fluvial-continental (upper Drumheller) environment.

The energy of flow culminated with the deposition of the thickest coarsening-upward unit CU1 (Figure 4), demonstrating that the depositional energy during the Lower Drumheller Coal Zone is generally higher than anytime later within the Horseshoe Canyon Formation.

Unit CU1, located at the top of the delta plain sequence, ends the prograding sequence and is capped by a thick coal package. This unit is interpreted as a succession of two to three vertically interrelated channels, representing the major channel association within the Horseshoe Canyon Formation of the study area. This widespread, coarse-clastic episode exhibits the highest level of flow energy and the maximum sediment supply, and also marks the division between two quiet depositional environments in the study area. The CU1 channel system may represent the only major channel across the study area.

The CU1 parasequence is thinner toward the north, where it was deposited by one channel and becomes thicker toward the south where it encompasses two to three aggrading channels. Within the western part of the area, unit CU1 is restricted to the centre. The CU1 channel migrated from south to north by three pulses along a southeast-northwest direction. Every pulse (caused by tectonism and/or climate) pushed the channel farther north, eventually covering the entire northern part of the study area.

The most consistent coal seam within the study area is the coalbed marker situated on top of unit CU1. This excellent marker represents a thick stack of seams with internal splitting and pinching-out relationships. Sedimentologically, the coal marker represents the final unit of a thick rhythmic upper delta plain sequence. Longer exposure created the conditions for a thicker peat accumulation with periodical flooding that generated the stack of seams. The sharp contact with the overlying sequence suggests a decrease of the deposition rate and a possible discontinuity, as indicated by the absence of the coal marker on one log from the central area (Figure 6).

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Upper Drumheller Coal Zone (Floodplain Sequence)

A major change in depositional conditions occurred after unit CU1 was deposited. This change was related to a change in the flow energy, and produced a thick floodplain succession with thin, fining-upward sequences. Based on log facies interpretation, the dominant trend of the floodplain sequences is aggradation. One to 13 cycles were observed in the Upper Drumheller Coal Zone.

The floodplain facies assemblage comprises thin and less continuous fining-upward sequences, which migrated northward in the same direction as unit CU1 channels. This succession represents tens of metres of siltstones, muds and fine-grained sandstone beds overlain by laminated sediments. Bentonites and coals are also present in the upper part of these sequences, characterized by upward-increasing gamma-ray trends. The coal beds show a cyclic occurrence. Moderate gamma-ray values (<160 API) dominate the lower part of these sequences, reflecting their interbedded character, whereas high-value spikes, representing siltstone, shale/mud, interbeds and partings, are common toward the top of the sequences (Figure 4). Usually, the base represents a sharp contact between the lower finer grain size and the upper coarser grain size sediments. Grain size decreases toward the top of the succession. Flood plain deposition is continuous above the Upper Drumheller Coal Zone; however, peat development becomes much less frequent.

The generally fining-upward sequences indicate that the average water velocities became progressively lower. The succession is interpreted as a cyclic aggrading floodplain.

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Coal Characterization

The data on the coal seams presented in the previous section provide the means to evaluate the available CBM resources in the Drumheller Coal Zone of the study area. Only coal and shaly coal intervals were used for the coal resource calculations. The ash content of coaly shale is too high for this rock to be a suitable coalbed methane reservoir. The density of the various lithologies can be estimated from the bulk density logs of wells in the study area. Based on this, a good density estimate for coal in the area is 1.4 g/cc, and for shaly coal is 1.6 g/cc. Tonnage per square kilometre can be calculated from these densities and coal thicknesses.

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Cleats

The orientation of fractures in coal (cleats) depends on the main tectonic directions. The face cleats are continuous for many metres, have significant permeability, and are usually parallel to the dip direction. Butt cleats are short, often curved and discontinuous, with less permeability, and are generally parallel to the strike direction. They frequently terminate against face cleats. No core or dipmeter data are available in the study area; therefore, cleats have not yet been observed. However, some predictions about cleat orientations can be made, based on inferred subsidence directions.

In the study area, the structure is quasi-planar and subhorizontal with depressed areas due to subsidence. The subsidence directions within the Lower Drumheller Coal Zone have a north-south trend (resulting from tectonic stress), possibly imposing a est-east direction on the face cleats. The Upper Drumheller Coal Zone has a northwest-southeast set of subsidence directions, which could result in face cleat directions of southwest-northeast.

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Coal Quality

The coal quality of the Drumheller Coal Zone of central Alberta as determined from shallow drillholes is presented by Nurkowski (1985). Coal ash averages 14%, with a volatile matter concentration (dry, ash-free basis) of 44%. Within and near the study area, Nurkowski (1985) reports higher ash averages (29%), with comparable volatile matter concentrations (45%, dry, ash-free basis). This indicates a rank of high volatile C bituminous. Beaton et al. (2002) report that deeper Drumheller coals range in rank from sub-bituminous B to high volatile A bituminous across the Alberta plains, with most of the coals being of high volatile C bituminous rank.

Vitrinite reflectance was determined on two drillhole chip samples from within the study area (Beaton et al. 2002; Rottenfusser et al., 1991). The two data points average 0.50% (Ro random), just within the boundary of high volatile bituminous C rank. Lower Horseshoe Canyon coals within the study area fall within a rank range just at the onset of thermogenic methane generation.

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Gas Content and CBM Potential

Thermogenic methane generation is initiated approximately at the rank of high volatile C bituminous coal and increases with increasing rank. Early stage biogenic gas may be present in the coals of the Alix area because the rank level is still relatively low. The possibility of these shallow coals acting as aquifers also suggests potential for late-stage, biogenic methane generation (Scott et al., 1994). The depth of the Drumheller coal in the Alix area ranges from 200 to 500 metres. At these depths, a reasonable estimate of gas content is 2 cc/g for coal and 1.5 cc/g for shaly coal. These gas content values are consistent with estimates by Beaton et al. (2002) for this area, which are based on random vitrinite reflectance observed in the area of 0.5%.

Total gas in place (GIP) equals the product of tonnage of coal and gas content per unit weight of coal. By using density in g/cc units and gas content in cc/g units, the formula for the GIP calculation simplifies to the product of the volume of coal and a constant. The constant is different for each type of coal and is calculated as the product of the density in g/cc and the gas content in cc/g (for example, for a coal with density of 1.4 g/cc and gas content of 4 cc/g, this constant is 5.6). Consequently, the formula we used is

GIP (Gas In Place) = Constant x Volume
Where: Constant = Density (in g/cc) x Gas content (in cc/g)

The results of this calculation are shown in Figure 18 and indicate gas content of 25 to 65 million cubic metres of methane per square kilometre (2.3-6 bcf/section). The map pattern is similar to the pattern shown by the total cumulative coal because the volume of coal is largely dependent on the thickness of coal. This map defines the areas of best potential for coalbed methane producibility in Range 23 in the western half of the study area. These areas, with estimated gas content of up to 65 million cubic metres of methane per square kilometre (6 bcf/section), could be good locations for test sites for enhanced CBM production. Specific test sites could be located in these areas, based on agreements with lease owners.

Figure 18. Gas content estimation in million cubic metres per square kilometre (File size: 154 KB)

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References

Allan, J.A. (1922): Geology of Drumheller Coal Field, Alberta; Scientific and Industrial Research Council of Alberta, Report 4, 72 p.

Allan, J. A. and Sanderson, J.O.G. (1945): Geology of the Red Deer and Rosebud Sheets, Alberta; Alberta Research Council, Report 13, 115 p.

Beaton, A., Pana, C., Chen, D., Wynne, D. and Langenberg, C.W. (2002): Coal and coalbed-methane potential of Upper Cretaceous-Tertiary Strata, Alberta Plains; Alberta Energy and Utilities Board, EUB/AGS, Earth Sciences Report 2002-06, CD-ROM, 77.5 MG.

Given, M.M. and Wall, J.H. (1971): Microfauna from the Upper Cretaceous Bearpaw Formation of south-central Alberta; Bulletin of Canadian Petroleum Geology, v.19, p.502-544.

Hamblin, A.P. (1998): Detailed core measured section of the Bearpaw/Horseshoe Canyon formations, C.P.O.G. Strathmore 7-12-25-25W4, east of Calgary, southern Alberta; Geological Survey of Canada, Open File 3589, 9 p.

Hamblin, A.P. (in press): The Horseshoe Canyon Formation in southern Alberta: stratigraphic architecture, sedimentology and resource potential; Geological Survey of Canada, Bulletin.

McCabe, P.J., Strobl, R.S., Macdonald, D.E., Nurkowski, J.R. and Bosman, A. (1989): Coal resource evaluation of the lower Horseshoe Canyon Formation and laterally equivalent strata, to a depth of 400 m, in the Alberta plains area; Alberta Research Council, Open File Report 1989-07.

Nurkowski, J.R. (1985): Coal quality and rank variation within Upper Cretaceous and Tertiary sediments, Alberta plains region' Alberta Research Council, Earth Science Report 85-1, 39 p.

Rahmani, R.A. (1988): Estuarine tidal channel and near-shore sedimentation of a late Cretaceous epicontinental sea, Drumheller, Alberta, Canada; in Tide-Influenced Sedimentary Environments and Facies, P.L. de Boer, A. van Gelder and S.D. Nio (editors), Riedel Publishing Company, p. 433-474.

Rottenfusser, B., Langenberg, W., Mandryk, G., Richardson, R., Fildes, B., Olic, J., Stewart, S., Eccles, R., Evans, C., Spelrem, M., Sprecher, B., Brulotte, M., Gentzis, T., Wynne, D. and Yuan, L.P. (1991): Regional evaluation of the coal bed methane potential in the Plains and Foothills of Alberta, stratigraphy and rank study; Alberta Research Council, Special Report 7, 126 p.

Scott, A.R., Kaiser, W.R. and Ayers, W.B. (1994): Thermogenic and secondary biogenic gases, San Juan Basin, Colorado and New Mexico - implications for coalbed gas producibility; AAPG Bulletin, v.78, p.1186-1209.

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Funding for this study was provided in part by a consortium led by the Alberta Research Council (ARC) on sustainable development of coalbed methane, with the purpose to identify suitable sites for enhanced CBM recovery (ECBM). The project team consisted of Willem Langenberg, Cristina Pana, Desmond Wynne, Darrell Cotterill, Andrew Beaton, Eric Grunsky, Jessica Delorme and Campbell Kidston.